Kassin et al. (2007) conducted a survey of investigators and their interrogation practices, and ...

Kassin et al. (2007) conducted a survey of investigators and their interrogation practices, and found that investigators rarely reported instances of using threats or physical intimidation. Why should this result be interpreted with caution?
a. The videos analyzed for this study only showed the behaviour of the suspect and not the investigator.
b. This was a self-report study and the investigators’ responses may have been biased to make themselves look better.
c. Much police intimidation occurs outside of the interrogation room, so not captured on film.
d. The researchers may have received a biased sample of only those officers who use “good cop” routines.
e. These techniques are considered illegal so police officers should not be using them in the first place.